Ambient air quality has become an increasingly important concern in recent years. Many air pollutant emission sources that were tolerated in years past are now facing regulations which force significant reductions or elimination of such emissions. One category of such emission sources is above-ground storage tanks for volatile liquids.
Although there are other types of above-ground volatile liquid storage tanks, one type of such tank in wide use is referred to as an internal floating-roof tank. This type of tank has a circular essentially flat bottom, a vertical cylindrical circular wall having a lower edge portion joined to the bottom and an external fixed roof joined to an upper portion of the tank wall. The tank has an internal floating roof adapted to float on a volatile liquid stored in the tank. The rim space between the floating roof periphery and the tank wall is sealed by one of several types of sealing means attached to and movable vertically simultaneously with the floating roof so as to reduce emissions into the tank vapor space between the floating roof, fixed roof and the tank wall. Some such seals are disclosed in the U.S. Pat. Nos.: Moyer 2,829,795; Harris et al. 2,968,420; Reese 3,075,668; Wissmiller 3,120,320; Moyer 3,136,444; and Bruening 4,406,377.
The tank fixed roof ca be wholly supported by the tank wall or it can be partially supported by the tank wall and partially supported by vertical columns in the tank. Since such columns must penetrate the floating roof, the holes in the floating roof are provided with seals to minimize flow of emissions through the holes into the tank vapor space. Bruening U.S. Pat. No. 4,243,151 discloses such a sealing system.
The described tank also has air vent means, generally positioned in the tank wall so as to be in communication with the tank vapor space when the floating roof floats on a predetermined maximum volume of volatile liquid in the tank.
The air vent means can comprise a multiplicity of spaced apart vent holes. Air flows inwardly into the tank vapor space through a portion of these vent holes dependent on ambient wind speed conditions. As the air flows in, the air mixes with emissions in the tank vapor space. The air laden with emissions then flows outwardly from the tank vapor space through the remainder of these vent holes into the ambient air, resulting in atmospheric emissions.
The air vent means can also comprise a single vent opening of essentially uniform width extending around substantially the entire circumference of the tank wall. Air flows inwardly into the tank vapor space through a portion of this vent opening that extends over part of the circumferential length of the vent opening dependent on ambient wind speed conditions. As the air flows in, the air mixes with emissions in the tank vapor space. The air laden with emissions then flows outwardly from the tank vapor space through the remaining circumferential length of the vent opening into the ambient air, resulting in atmospheric emissions.
American Petroleum Institute Standard 650, Appen. dix H, describes the venting requirements for internal floating-roof tanks. This standard requires that shell vents be provided which have a total vent area, A, of 0.2 square feet of vent area per foot of tank diameter, with a minimum of four shell vents. The maximum spacing between shell vents may not exceed 32 feet. In addition, a roof vent of 50 square inches minimum area is required on the fixed roof.
The combination of shell vents and roof vent provide sufficient natural ventilation of the tank vapor space to minimize the possibility of creating a combustible mixture in the tank vapor space for volatile liquids with a vapor pressure up to 11.0 pounds per square inch absolute. Thus, internal floating.roof tanks inherently provide a high degree of safety in service for the storage of volatile liquids.
The emissions may be divided into standing storage emissions and working emissions. Standing storage emissions are the evaporative loss of volatile liquid stock vapor that occurs without any change in the liquid level in the tank. These emissions result from the floating roof rim seal system, floating roof fittings and floating roof seams, if not welded. Working emissions are the evaporative loss of stock vapor resulting from a decrease in liquid level in the tank. As stock is withdrawn from the tank and the floating roof descends, stock clings to the inside surface of the tank wall and evaporates into the tank vapor space. All emissions first accumulate in the tank vapor space, where they mix with ventilation air that enters the tank vapor space through a portion of the air vent means which can comprise some of the wall vent holes. Secondly, the emissions laden air leaves the tank vapor space through the remaining portion of the air vent means which can comprise the remaining wall vent holes, resulting in the emissions entering the atmosphere. It is the release of such emissions to the atmosphere which must be reduced or eliminated to comply with expected regulations reducing environmental pollution.